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A nonNewtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity of nonNewtonian fluids is not independent of shear rate or shear rate history. However, there are some nonNewtonian fluids with shearindependent viscosity, that nonetheless exhibit normal stressdifferences or other nonNewtonian behaviour. Many polymer solutions and molten polymers are nonNewtonian fluids, as are many commonly found substances such as ketchup, custard, toothpaste, starch suspensions, paint, blood, and shampoo. In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, the constant of proportionality being the coefficient of viscosity. In a nonNewtonian fluid, the relation between the shear stress and the shear rate is different, and can even be timedependent. Therefore a constant coefficient of viscosity cannot be defined.
Although the concept of viscosity is commonly used to characterize a material, it can be inadequate to describe the mechanical behavior of a substance, particularly nonNewtonian fluids. They are best studied through several other rheological properties which relate the relations between the stress and strain rate tensors under many different flow conditions, such as oscillatory shear, or extensional flow which are measured using different devices or rheometers. The properties are better studied using tensorvalued constitutive equations, which are common in the field of continuum mechanics.
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